Currently, some optical sampling oscilloscopes used for observing ultrahigh-speed signal light have been commercialized. An optical sampling oscilloscope is a measurement instrument for accurately observing and evaluating a waveform of an optical signal at a high speed that cannot be observed by means of electricity.
A bit rate of the optical signal has recently reached 40 Gb/s, and, in an attempt to realize a future high-speed system, an element technology is under development to enable optical signal transmission at 160 Gb/s or higher. Therefore, an optical sampling oscilloscope with a higher time resolution is demanded. Accordingly, generation of short pulse light, as sampling light, having a narrower pulse width than that of signal light is essential.
If the sampling pulse width is narrow, sampling points per pulse can be increased, thus enabling the optical sampling oscilloscope to measure a waveform closer to the actual optical waveform (increased time resolution). However, by implementing an optical gate in a time domain, optical noise could increase in the optical waveform output after the optical sampling is applied, compared with optical noise in the original input optical pulse. In other words, accuracy of the optical waveform measurement and quality of the optical pulse (optical noise, Q-value) are in trade-off relationship.
Therefore, in an optical sampling oscilloscope, understanding of intrinsic optical noise of the optical sampling oscilloscope is an important measurement parameter in defining characteristics of the measurement instrument, as well as in defining conditions that the pulse of an optical waveform is measured under.
A typical method for measuring the optical noise is a probe method used for measuring noise in signal light amplified by an erbium-doped fiber amplifier (EDFA) as depicted in FIG. 5. In an EDFA, an optical spectrum form almost does not change between an input end and an output end as depicted in FIG. 6. Therefore, noise in an optical signal can be defined by measuring an optical noise level and an optical signal level, and subtracting the optical noise level from the optical signal level. This is because a current bit rate of the signal light amplified by the EDFA does not require a broad bandwidth, and power level of the signal light is not high enough to induce a nonlinear optical effect that causes the optical signal spectrum to change.
An example of such an optical sampling oscilloscope is described in detail in Japanese Laid-open Patent Publication No. 2006-184851.
However, in an optical sampling oscilloscope using a nonlinear medium, an optical signal to be measured is at a high bit rate (up to 160 Gb/s), and a bandwidth of the signal light is broad. Moreover, as depicted in FIG. 7, due to nonlinear optical effects, such as four wave mixing, on the sampling light or the signal light, the signal light broadens further in a wavelength domain. Therefore, to avoid being influenced by adjacent signal light, the sampled signal light needs to be filtered using an optical filter at the output end. Thus, it becomes difficult to determine the level of the optical noise. Moreover, the spectrum of the optical signal differs greatly before and after being input to the nonlinear medium. For these reasons, an optical sampling oscilloscope using a nonlinear medium has a problem that it is difficult to measure the optical noise level accurately in the wavelength domain.
Furthermore, parameters indicating the quality of the optical pulse, such as a Q-value of the optical pulse, are basically calculated from a measurement of the optical noise. Therefore, with commercialized optical sampling oscilloscopes, it has been difficult to specify and guarantee the quality of a pulse, and to understand the intrinsic optical noise characteristics of the optical sampling oscilloscope, such characteristics being essential in a measurement instrument.